Conventionally, for example, an end mill used for die cutting, etc. includes an end mill body made of a hard material, such as cemented carbide, and formed in a cylindrical rod shape. The end mill body includes a shank portion, by which devices, such as the chuck of a machine tool, can hold the end mill body, placed at the rear end side in the direction of the axis; and a cutting edge portion, where cutting edges for cutting a workpiece are provided, placed in closer to the top side than the shank portion in the direction of the axis. In such an end mill, multiple flutes having a helical shape around an axis are formed in the periphery of the cutting edge portion from the top side toward the rear end side. Further, peripheral cutting edges (cutting edges) are formed along intersecting ridgeline portions (outer peripheral side ridge portions) between the wall surfaces of the flutes facing to the front side in an end mill rotating direction and peripheral surface facing radially outward. Additionally, end cutting edges are formed along intersecting ridgeline portions between the wall surfaces of the flutes facing to the front side in the end mill rotating direction and the top face of the end mill body.
To hold the above described end mill in a machine tool, a chuck of the machine tool grips the shank portion placed on the rear end side of the end mill body in a cantilevered state. Further, when rotating the end mill on the axis at high speed, and also then feeding the end mill in a direction intersecting with the axis; peripheral cutting edges of the cutting edge portion cut into a workpiece to machine it.
In order to prolong the tool life of this end mill and also to improve the cutting performance of this end mill, it is important to improve the fracture resistance of the cutting edges. However, if the cutting edges are excessively sharp for cutting a workpiece, cutting force may concentrate in the cutting edges, and then fracture may occur thereon. If the cutting edges with such fracture continue cutting; a problem arises in which the cutting performance becomes remarkably poor. To improve the fracture resistance, for example, Patent Documents 1 and 2 mention end mills in which straight or round shaped honing is made along the cutting edges.